Statistics show that nearly 70% of adults ages 35 to 44 have lost at least one permanent tooth to an accident, gum disease, a failed root canal, or tooth decay. By age 74, it is reported that 26% of adults have lost all of their permanent teeth. Both the increasing aging population and a growing awareness for oral health and aesthetics have led to the growth of dental implant surgery. A dental implant is a permanent post anchored to the jawbone and topped with a prosthetic (implant abutment and synthetic crown or bridge) that can be permanently attached to the post. Single teeth or an entire arch of teeth may be effectively replaced with dental implants and attached prosthetics, which can last for significant periods of time with routine maintenance. Dental implant surgery is now considered to be the fastest growing area in dentistry.
Dental implant posts are typically made of titanium or titanium alloys, which generally are anchored to bone via osseointegration (intimate physical contact between the synthetic implant and the surrounding bone). Traditionally, metallic prosthetic components have been used to restore implants. However, recent commercial development has focused on alternative materials, especially ceramics. Ceramics provide high strength as well as the natural look of real teeth. In many cases, ceramics have higher wear resistance, corrosion resistance, toughness, and strength than metals and metal alloys. In particular, recent research has focused on high strength ceramics such as alumina and zirconia. These materials provide better fracture resistance and long-term durability than traditional porcelain and other ceramics.
The methods for attaching a substrate (natural tissue like tooth structure or implant abutment) to a prosthetic restorative may be micromechanical, or may additionally include chemical bonding through silanation or other surface treatment techniques. In some applications, adhesive bonding is not required and the ceramic material may be placed and affixed using conventional cements that rely on micromechanical retention. Micromechanical retention may be achieved in some cases by merely roughening the surfaces of the substrate or the restorative. However, these conventional cementation techniques do not provide the high bond strength required for some applications. In such applications, good adhesion is often important for high retention, prevention of microleakage, and increased fracture and fatigue resistance, and may be provided by resin-based cements used in conjunction with intermediate adhesion promoters, like dental silanes. Strong resin bonding relies on micromechanical interlocking as well as adhesive chemical bonding to the ceramic surface and requires a combination of surface roughening and chemical functionalization for efficient attachment.
Surface roughening may be achieved by grinding, abrasion with diamond rotary instruments, surface abrasion with alumina particles, acid etching with acids such as hydrofluoric acid (HF), or a combination of these techniques. Adhesive chemical bonding is commonly achieved through a two-step process, which initially involves treating the implant or restorative with a silane coupling agent. The silane coupling agents are organic compounds that contain silicon atoms, are similar to orthoesters in structure, and may display dual reactivity. Silanes typically contain one or more alkoxy groups, wherein the alkoxy groups can react with an inorganic substrate. The other end of the molecule is organically functionalized, for example, with a vinyl, allyl, isocyanate, or amino group, and can polymerize with an organic matrix such as a methacrylate. The next step of achieving the adhesive chemical bonding is using an organic resin-based cement to react with the organically functionalized silane to affix adherends.
This adhesive chemical bonding, which is required for many dental applications, is not applicable to high strength ceramic materials. Because of the composition and physical properties of high-strength ceramics, they are not easily etched or chemically functionalized using conventional treatments. Traditional silane chemistry is not effective with high strength ceramics because such materials are more chemically stable (inert) than silica-containing materials and are not as easily hydrolyzed. Furthermore, due to their hardness and strength, the surfaces of high strength ceramics are not easily roughened. Acid etchants such as HF do not sufficiently roughen the surface. These materials may be roughened only by very aggressive mechanical abrasion methods, which may create fatigue-enhancing surface flaws.
One method that can be used to provide adhesive chemical bonding of high strength ceramics requires surface abrasion with alumina particles coated with silica. The alumina particles impact the surface, transferring a thin silica layer via a tribochemical process, which allows for chemical bonding to a silane coupling agent, which can then bond to a resin-based cement. However, this method is a relatively complicated procedure and does not produce bond strengths as high as those reported for silane-bonded porcelain. In addition, air particle abrasion may be particularly unsuitable for zirconia-based materials, as it is likely to generate micro-fractures which could lead to premature, catastrophic failure.
Alternatively, the use of phosphoric acid primers or phosphate-modified resin cements has been shown to produce silane-like adhesion through similar types of hydrolyzation-driven chemistry. However, the bond strengths reported are generally even lower than those reported for the tribochemical silica coating in combination with silane and resin cement. One recent study has shown increased bond strength using selective infiltration etching and novel silane-based zirconia primers. See Aboushelib M N, Matinlinna J P, Salameh Z, Ounsi H., Innovations in Bonding Zirconia-Based Materials: Part I. Dent. Mat. 2008; 24: 1268-1272. However, the available approaches for adhesive bonding of high strength ceramics are not adequate for all clinical applications and their long-term efficacy is currently unknown. A non-destructive, simple method for treating high strength ceramic surfaces would be desirable to render the high strength ceramic surfaces suitable for use with existing adhesive bonding techniques.